Composition dependences of glass-transition temperature, optical gap, photodarkening, and photoinduced volume change in xGeO(2)-(100-x)GeS2 oxy-chalcogenide glasses have been studied comparatively. The transition temperature and optical gap are nearly constant at 0 <= x less than or similar to 80 mol %, while the photodarkening and volume expansion appear only at x less than or similar to 60. Implications of these results are discussed.